Polypropiolactone, termed “PPL” for the purposes of this application, is a biodegradable polymer that can be useful material in many manufacturing and industrial applications. The physical and chemical characteristics of PPL provide for safer transportation and storage over extended periods of time with decreased quality concerns. PPL is also a useful precursor because the polymer may undergo a chemical process known as thermolysis to produce acrylic acid.
Generally, thermolysis is a chemical decomposition reaction caused by heat. Thermolysis of PPL may proceed by two known reaction mechanisms. A first reaction mechanism, known as unzipping, includes a PPL polymer with a chain length equal to (n) that decomposes into a PPL polymer with a chain length (n−1) and a molecule of acrylic acid. The second reaction mechanism, known as internal chain scission, includes a PPL polymer with a chain length (n) decomposes into a PPL polymer with a chain length (n−x) and a PPL polymer with a chain length (x), where (x) is greater than or equal to 2.
Under certain reaction conditions, acrylic acid may be susceptible to auto-polymerization. In one auto-polymerization reaction, a first molecule of acrylic acid is added to a second molecule of acrylic acid to form a di-acrylic acid ester, which is identical to a PPL polymer with a chain length of 2. There is no known inhibitor which will prevent the addition of one molecule of acrylic acid to another. However, the di-acrylic acid ester may readily undergo thermolysis and decompose back into two molecules of acrylic acid. In a second auto-polymerization reaction, multiple molecules of acrylic acid undergo free radical polymerization to form chains of polyacrylate. These larger chains of polyacrylic acid cannot convert back into individual molecules of acrylic acid under thermolysis conditions.
Radical polymerization of acrylic acid may be limited with the use of certain known inhibitors. However, these radical polymerization inhibitors may be costly, inefficient, and/or difficult to source. Additionally, conventional thermolysis reactors may not efficiently utilize reactants or may not allow for ready recovery of acrylic acid vapor effluent. There exists a need for improved apparatus and methods for thermolyzing PPL by catalyzing thermolysis while reducing and/or limiting formation of polyacrylic acid with less expensive, more efficient, and more easily sourced materials. The present invention satisfies this need by providing reactor apparatus and processes for thermolysis of PPL which may be configured to maintain concentrations of active salt.